1. Field of the Invention
The invention concerns a method of producing a metallic thin film magnetic disk in which a metal undercoat, in particular of chromium, is initially applied to a substrate and a metallic magnetic layer is subsequently vacuum-applied at an angle of between about 50.degree. and 75.degree. obliquely to the substrate normal.
2. Description of the Prior Art
U.K. Pat. No. 1,439,869 shows a magnetic record carrier having a metallic intermediate layer or an undercoat of chromium, titanium, manganese or vanadium applied to a substrate consisting, for example, of an aluminum alloy. Subsequently, a very thin metallic magnetic recording layer of an iron-cobalt alloy is applied to this undercoat. Both the undercoat and the magnetic thin film are applied in a vacuum which must be higher than 0.01 .mu.bar. The magnetic layer consisting of iron-cobalt alloy is vapor deposited at an angle of incidence of about 60.degree. to the substrate normal.
The oblique vapor deposition leads to a directional anisotropy of the magnetic film and thus to an increase in the coercive field strength. To ensure that the individual particles of the magnetic material are obliquely arranged on the record carrier, the vapor pressure must be such that the mean free path length corresponds at least to the spacing between the vapor deposition source and the substrate, i.e., on its way to the record carrier, the particle exiting from the source must not collide with a gas particle. This requirement is met by the minimum vacuum value of 0.01 .mu.bar mentioned in the U.K. patent mentioned above.
When a multi-component alloy is vapor deposited, difficulties and instabilities occur because the vapor pressures of the materials, and thus the vapor deposition rates, are non-uniform. The composition of the vapor deposited magnetic material, for example, fluctuates irrespective of whether vapor deposition is effected from several sources, each with one material component, or from one source with a composite alloy material. The latter type of alloy is among those to which the rod feed technique is applied, for which purpose the alloy to be vapor deposited exists in rod form. With this technique, the rod is heated at its end, forming a melting pool, and advanced in accordance with the vapor deposition rate. During this, inhomogeneities in the material to be deposited and the formation of slag on the surface of the melting pool lead to fluctuations in the rates and concentration of the finished magnetic film.
A magnetic thin-film record carrier with a magnetic metal layer and a metallic undercoat is also shown in U.K. Pat. No. 1,408,753. The magnetic layer consists of iron-cobalt and is vapor deposited obliquely to the substrate normal at an angle of between 50.degree. and 75.degree., preferably of about 60.degree., at a vacuum higher than about 0.1 .mu.bar. The metallic undercoat containing, for example chromium, is not obliquely vapor deposited. With regard to the undercoat containing, for example, pure chromium and essentially chromium oxide, this patent states that the undercoat, because of its hardness as an oxide or nitride layer, leads to an increase in the coercive field strength, and that the pure chromium contained in it does not adversely affect the magnetic properties of the iron-cobalt layer which is subsequently obliquely vapor deposited.
In U.S. Pat. No. 3,850,690 a method is described for producing a metallic thin-film magnetic disk with different coercive field strengths at the inner and outer diameter, respectively. On both sides of a vertically arranged disk substrate, one vapor deposition source each for an iron-cobalt alloy is so arranged outside the cylinder described by the substrate during its rotation that the angle of incidence between the vapor beam and the vertical of the substrate is smaller at the outer diameter than at the inner diameter. The disk substrate is rotated and the thickness of the vapor deposited film is controlled by shields arranged on both sides. With this method, too, the magnetic film is obliquely vapor deposited at a vacuum higher than 0.01 .mu.bar.
U.S. Pat. No. 4,245,008 shows the fabrication of a thin film disk by first sputtering an underlayer on a substrate and then sputtering a magnetic layer of CoCrFe on the underlayer.
The prior art shows quite clearly that the minimum vacuum pressure range for oblique deposition should be about 0.1 .mu.bar, because of the mean free path length necessary for the vapor particles. For this reason, magnetic materials were previously applied exclusively by the vapor deposition method. This method is however elaborate and often unstable, thus not yielding optimum results.